The present invention relates generally to electronic driver circuits for power light-emitting diodes (LEDs). An LED operates in response to a direct current flowing through the device from the anode to the cathode. Above a current threshold, the LED will begin emitting light at an intensity determined by the magnitude of the current. In a typical lighting application, a plurality of LEDs are connected in series so that a common current flows through the LEDs to cause each of the LEDs to illuminate with substantially the same intensities to provide a uniform lighting effect. The current through the LEDs is provided by an electronic LED driver circuit that provides an output voltage sufficient to cause the current to flow through the series connected LEDs. The LED driver circuit controls the current to a magnitude selected to provide the desired illumination intensity. The magnitude may be controlled by a dimmer circuit to allow the magnitude to be changed to thereby control the illumination intensity produced by the LEDs.
In a typical fixed (e.g., non-portable) application, an electronic LED driver circuit receives AC power from a conventional AC supply (e.g., by hard wiring an LED lighting fixture to the electrical wiring of a building or by plugging an LED lighting fixture into a conventional outlet). The LED driver circuit converts the AC power to DC power, and the DC power is connected to the LEDs.
An exemplary LED driver circuit 100 is illustrated in FIG. 1. The driver circuit receives AC power from an AC source 110. The AC power is coupled to the inputs 122, 124 of a full-wave bridge rectifier 120. The bridge rectifier includes a first bridge diode 130, a second bridge diode 132, a third bridge diode 134 and a fourth bridge diode 136. The four bridge diodes operate in a conventional manner to convert the AC input power to a DC output voltage. The DC output voltage is provided between a first (+) output 140 and a second (−) output 142 of the bridge rectifier. The voltage on the first (+) output of the bridge rectifier is identified as VBRIDGE, and is provided on a VBRIDGE bus 144. The second (−) output of the bridge rectifier is connected to a common DC ground reference of the circuit 146.
The positive DC output voltage (VBRIDGE) on the first (+) output 140 of the bridge rectifier 120 is connected to a first terminal of a bridge load resistor 150. A second terminal of bridge load resistor 150 is connected to the DC ground reference 146. The bridge load resistor 150 operates as a discharge resistor to discharge various capacitors in the circuit when AC power is no longer applied to the inputs 122, 124 of the full-wave bridge rectifier 120.
The positive DC output voltage (VBRIDGE) is also connected to an input 162 of a power factor correction (PFC) circuit 160. The PFC circuit has an output terminal 164 that provides a DC voltage (VRAIL) on a voltage bus 166. A voltage rail (VRAIL) filter capacitor 168 is connected between the voltage bus and the ground reference. The VRAIL voltage bus is connected to the voltage input of a DC-DC converter stage 170. The DC-DC converter stage 170 provides an output voltage (VLED) to an LED load 172. Although represented as a single load, the LED load may include a plurality of LEDs connected in series or connected in a series/parallel combination. The VLED output voltage causes current to flow through the LEDs to illuminate the LEDs in a conventional manner.
The DC-DC converter stage causes harmonics on the VRAIL voltage bus 166. If the DC-DC converter were connected directly to the output of the full-wave bridge rectifier 120, the harmonics would reduce the power factor of AC power coupled to the inputs 122, 124 of the full-wave bridge rectifier. The PFC circuit 160 isolates the VRAIL voltage bus from the VBRIDGE voltage bus. The PFC circuit operates in a conventional manner to cause the overall load between the first and second outputs of the bridge rectifier to have a greater effective power factor (e.g., a power factor closer to an ideal power factor of 1). The PFC circuit may comprise passive components or active devices. For example, in one embodiment, the PFC circuit may be a conventional power factor control circuit based on the STMicroelectronics L6562 Transition-Mode PFC Controller.
As shown in FIG. 1, the DC-DC converter stage 170 includes an integrated circuit controller (IC CTRL) 180. In the illustrated embodiment, the controller may be an L6384 High Voltage Half-Bridge Driver, which is commercially available from STMicroelectronics. The controller drives a first semiconductor switching element (e.g., a MOSFET) 182 and a second switching element (e.g., a MOSFET 184, which are connected in series between the VRAIL voltage bus 166 and the circuit ground reference 146. The first and second switching elements are connected at a common node 186. The controller has an input (IN) that receives a periodic signal (f) from a signal source 188, which may be a fixed frequency signal source or a variable frequency signal source. The controller drives the two switching elements in response to the periodic signal. When the first switching element is turned on, the common node is pulled up to the voltage VRAIL. When the second switching element is turned on, the common node is pulled down to ground. The two switching elements are operated in a conventional manner at a selected frequency and with selected duty cycles to produce a switched DC voltage at the common node that alternates between VRAIL and ground.
The common node 186 between the two switching elements 182, 184 is connected to a first input 192 of a power tank circuit 190. A second input 194 of the power tank is connected to the ground reference 146. The power tank circuit has an input portion 196 and an output portion 198. The combination of the power tank circuit, the controller 180 and the switching elements 184, 186 operate as a resonant DC-DC converter to convert the VRAIL voltage on the bus 166 to the VLED voltage applied to the LED load 172.
The input portion 196 of the power tank circuit 190 includes a resonant inductor 200 and a resonant capacitor 202, which are connected in series between the common node 186 and the ground reference 146. The inductance of the resonant inductor and the capacitance of the resonant capacitor are selected to resonate at the switching frequency of the controller 180 such that the switched DC voltage on the common node 180 causes an AC voltage with a DC offset component to be produced across the resonant circuit capacitor. The switching frequency of the controller is variable to adjust the magnitude of the AC voltage across the resonant circuit capacitor. For example, when the switching frequency is reduced below the resonant frequency or increased above the resonant frequency, the voltage across the capacitor decreases. Accordingly, by adjusting the switching frequency, the voltage can be selectively reduced to reduce the current through the LED load 172 and thereby cause the light produced by the LED load to be dimmed.
A DC blocking capacitor 210 and the primary winding 214 of a transformer 212 are connected in series across the resonant circuit capacitor 202 to cause only the AC component of the voltage across the resonant circuit capacitor to be coupled to the primary winding. The DC blocking capacitor 210 prevents DC current from passing through the primary winding.
In the output portion 198 of the power tank circuit 190, the transformer 212 has a center-tapped secondary winding 216. The secondary winding has a first winding half 218 and a second winding half 220. The first winding half is connected between a common node 222 and a first output terminal 224. The second winding half is connected between the common node and a second output terminal 226. The first output terminal is connected to the anode of a first rectifying diode 232 in a full-wave rectifier 230. The second output terminal is connected to the anode of a second rectifying diode 234 in the full-wave rectifier. The cathodes of the two rectifying diodes are connected together at a rectifier output node 236. The rectifier output node is coupled to a first (+) output 240 of the power tank circuit 190. The common node of the secondary winding is connected to a second (−) output 242 of the power tank circuit.
The voltage produced between the first (+) output 240 and the second (−) output 242 of the power tank circuit 190 is applied across a load capacitor 250 and across the LED load 172. The voltage is identified as VLED. The load capacitor filters out the high frequency ripple of the VLED voltage. Although shown outside the power tank circuit, the load capacitor may also be considered to be part of the power tank circuit.
A charge pump circuit 260 is connected to the common node 186 between the two switching elements 182, 184. The charge pump circuit includes a charge pump input capacitor 262 having a first terminal connected to the common node. The second terminal of the charge pump input capacitor is connected to the anode of a first charge pump diode 264 and to the cathode of a second charge pump diode 266. The anode of the second charge pump diode is connected to the DC ground reference 146. The cathode of the first charge pump diode is connected to a first terminal of a VCC filter capacitor 270 and to the power input (VCC) of the integrated circuit controller 180 at a node 272 identified as VCC. A second terminal of the VCC filter capacitor is connected to the DC ground reference 146. The second charge pump diode is a Zener diode having a voltage rating selected to clamp the voltage applied to the VCC node via the first charge pump diode during the positive going (+dv/dt) half of each switching cycle at the common node. The second charge pump diode also provides a discharge path for the charge pump input capacitor during the negative going (−dv/dt) half of each switching cycle at the common node.
The VCC node 272 is also connected to a first terminal of a power input resistor 280. A second terminal of the power input resistor is connected to the VBRIDGE bus 144. As described below, the power input resistor 280 operates as a passive voltage supply circuit that receives power from the VBRIDGE bus and that provides a first charging voltage via one or more passive components.
The power input resistor 280 and the charge pump circuit 260 both supply power to the VCC node 272 and thus to the VCC power input of the controller 180. Upon initial startup, the VCC filter capacitor 270 is charged from the VBRIDGE bus 144 through the power input resistor. Thus, the power input resistor (the passive voltage source) operates as a first charging voltage source to the VCC filter capacitor. The resistance of the power input resistor is selected to charge the capacitor at a relatively slow rate. For example, in one embodiment, the resistor has a resistance of approximately 150 ohms, and the VCC filter capacitor has a capacitance of approximately 2.2 microfarads.
The VCC filter capacitor 270 continues to charge through the power input resistor 280 until the voltage on the VCC node 272 reaches a threshold voltage sufficient to initiate the operation of the controller 180. When the threshold voltage is reached, the controller begins to operate in a conventional manner as described above to switch the two switching elements 182, 184 to generate the switched DC voltage on the common node 186. The switched DC voltage is coupled through the charge pump capacitor 262 and the first charge pump diode 264 to provide a second charging voltage source to charge the VCC filter capacitor. Together, the two switching elements and the charge pump 260 operate as an active voltage source to charge the VCC filter capacitor when the DC-DC converter 170 is operating. The power provided to the VCC filter capacitor via the charge pump is not dissipated by a dropping resistor or other resistive element. Thus, during normal operation, the voltage on the VCC node to maintain the charge on the VCC filter capacitor is provided primarily by current provided by the charge pump.
The above-described LED drive circuit 100 provides current to the LED load 172 as long as the AC source 110 continues to provide input power. When the input power is lost (e.g., the AC source turns off or is disconnected), the voltage on the VRAIL bus 166 is maintained by the VRAIL capacitor 168 as the VRAIL filter capacitor starts to discharge slowly. The VRAIL filter capacitor discharges at a rate determined by the capacitance of the VRAIL filter capacitor and the current provided to the LED load via the DC-DC converter 170. A greater load (e.g., more current flowing through the LED load) causes the VRAIL filter capacitor to discharge faster; and a lower load causes the VRAIL filter capacitor discharged slower. Thus, when the LED load is operating in a dimmed condition with a lower current flowing through the LED load, the VRAIL filter capacitor discharges slowly such that the controller 180 continues to operate for a substantial time (e.g., up to at least a few hundred milliseconds).
The foregoing effect is illustrated in FIG. 2. In the uppermost waveform of FIG. 2, the voltage VRAIL is shown as being substantially constant from a time t0 until a time t1. At the time t1, the AC input power is lost (e.g., by turning off a wall switch or the like). Although the AC source 110 is no longer providing a voltage to the input of the PFC 160 via the full-wave bridge rectifier 120, the voltage VRAIL does not decrease immediately. Rather, the voltage decreases slowly as the VRAIL filter capacitor 168 discharges through the LED load 172. Although the discharge is illustrated as a straight line, it should be understood that the discharge may be nonlinear (e.g., exponential). For simplification, nonlinear voltage and current values are represented herein as straight line segments.
During the initial discharge of the VRAIL filter capacitor 168, the controller 180 and the switching elements 182, 184 continue to operate to provide an AC voltage at the common node 186. The charge pump 260 continues to operate to provide VCC to the power input of the controller. The power tank 190 continues to provide the voltage VLED on the rectifier output node 236 connected to the LED load 172; however, as shown in the middle waveform in FIG. 2, the voltage VLED also decreases in response to the decreasing voltage VRAIL. The decreasing voltage VLED causes the current flowing through the LED load to decrease as represented by a current ILED shown in the lowermost waveform in FIG. 2. The decreasing current causes the light produced by the LED load to gradually dim.
As further shown in FIG. 2, the voltage VRAIL continues to decrease as the VRAIL filter capacitor 166 continues to discharge. The voltage VLED also continues to decrease in response to the decreasing voltage VRAIL; however, at a time t2, the voltage VLED decreases below a threshold voltage VLEDTH and is no longer sufficient to maintain current flow through the series-connected LEDs in the LED load 172. Thus, the LEDs in the LED load turn off at the time t2, and current no longer flows through the LEDs as illustrated the current ILED decreasing to zero at the time t2.
Although the foregoing operation would not be an issue if the LEDs in the LED load 172 remained off, the controller 180 continues to switch the switching elements 182, 184 and thus continues to maintain an AC voltage on the common node 186 at the input to the power tank circuit 190. The power tank circuit continues to provide the DC voltage VLED to the load capacitor 240 across the LED load. Because the LEDs in the LED load are no longer conducting and thus present no load to the power tank circuit, the voltage across the load capacitor starts to increase rapidly. This rapid increase is represented as a sharp voltage spike 290 between the time t2 and a time t3. The voltage spike has a magnitude substantially greater than the threshold voltage VLEDTH of the series-connected LEDs in the LED load. Thus, a large current—represented by a spike 292 in the current waveform LED in FIG. 2—flows through the LED load and causes a visible flash of light from the LEDs in the LED load. The sudden flash of light after the LEDs have apparently turned off is annoying to occupants of an area illuminated by the LEDs and may suggest to the occupants that the LED fixture has failed.